CN101300758A - Methods and apparatus for optical transmission of digital signals - Google Patents
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Abstract
Description
技术领域technical field
本发明总的来说涉及光通信,更具体地涉及用于调制和传输数字信号的方法和设备,其在包括色散的光路径的通信系统中特别有优势。The present invention relates generally to optical communications, and more particularly to methods and apparatus for modulating and transmitting digital signals, which are particularly advantageous in communications systems involving dispersive optical paths.
技术背景technical background
信息信号被调制到光载波(optical carrier)上的光传输广泛应用于现代通信系统中。例如,长距离传输网络在每个光纤上使用一个或多个光载波或波长,采用单模光纤以非常高的比特率传输数字信息。而且,已知在更短的距离和以更低的比特率的光传输,例如在用户接入网或局域数据网中的光传输,其采用单模或多模光纤。此外,自由空间光传输路径可用于视距通信(line-of-sight communication)和/或用于提供装置的无线互连的简单方式,所述装置例如为计算机和外围设备、个人数字助理(PDA)以及其它一般使用红外光源和检测器的便携式装置。Optical transmission in which information signals are modulated onto optical carriers is widely used in modern communication systems. For example, long-distance transmission networks use single-mode fiber to transmit digital information at very high bit rates using one or more optical carriers, or wavelengths, on each fiber. Furthermore, optical transmission over shorter distances and at lower bit rates is known, for example in subscriber access networks or local area data networks, using single-mode or multimode optical fibres. Furthermore, free-space optical transmission paths can be used for line-of-sight communication and/or for providing an easy way of wirelessly interconnecting devices such as computers and peripherals, personal digital assistants (PDAs) ) and other portable devices that generally use infrared light sources and detectors.
因此,现在的兴趣是提供使得该通信能够在更长的传输距离上和/或以更高的可靠性、效率和/或容量进行的光通信方法和设备。It is therefore of interest now to provide optical communication methods and devices that enable this communication over longer transmission distances and/or with greater reliability, efficiency and/or capacity.
然而不管是通过单模还是多模光纤还是通过自由空间的光传输路径都可在所传输的光信号中引入失真。而且,用于发送和接收光信号的装置是另外的可能的信号失真源。失真的常见形式是响应于通信通道的频率相关的变化,这可导致在特定频率范围上和/或在具体频率处信号幅度受到抑制(或衰减),以及接收信号作为频率函数的延迟或相位的变化。该失真可源自在传输系统中使用的各种电子和光学装置的频率特性,以及光传输路径本身的特性。However, optical transmission paths, whether through single-mode or multimode fiber or through free space, can introduce distortion in the transmitted optical signal. Also, the devices used to transmit and receive optical signals are another possible source of signal distortion. Common forms of distortion are frequency-dependent changes in response to the communication channel, which can result in suppression (or attenuation) of signal amplitude over and/or at specific frequencies, and delay or phase delay of the received signal as a function of frequency Variety. This distortion can originate from the frequency characteristics of various electronic and optical devices used in the transmission system, as well as the characteristics of the optical transmission path itself.
具体而言,各种光路径一般呈现出色散的形式,由光路径上传输的光信号或其分量经历的延迟的变化表征。自由空间光路径可呈现多路径效应,在多路径效应中从光源发送的信号通过许多不同的空间路径到达远程光接收机,所述空间路径可包括直接(视距)路径以及包括周围环境中各个表面反射的一个或多个路径。因此,所接收的光信号的不同分量经历不同的传输延迟,导致在所接收的光信号中的时间展宽(time spread),而这是一种形式的色散。Specifically, various optical paths generally exhibit a form of chromatic dispersion, characterized by changes in the delay experienced by an optical signal or components thereof traveling on the optical path. A free-space optical path can exhibit multipath effects, where a signal sent from a light source travels many different spatial paths to a remote optical receiver, which can include the direct (line-of-sight) path as well as various signals in the surrounding environment. One or more paths reflected by the surface. Thus, different components of the received optical signal experience different propagation delays, resulting in time spread in the received optical signal, which is a form of chromatic dispersion.
在多模光纤中,耦合到光纤中的信号引起由光纤支持的许多横向光模式(transverse optical mode),每个横向光模式由不同的群速度(groupvelocity)表征。因此,耦合到不同横向模式的传输信号的分量沿着多模光纤经历不同的传输延迟,这又导致接收信号的色散。In a multimode fiber, a signal coupled into the fiber induces many transverse optical modes supported by the fiber, each characterized by a different group velocity. Consequently, components of the transmitted signal coupled to different transverse modes experience different propagation delays along the multimode fiber, which in turn leads to chromatic dispersion of the received signal.
在自由空间光传输系统中的多路径传播以及多模光纤内的模态色散(modal dispersion)所导致的色散可以是相对静态的或时变的,且可导致所接收信号中的相对应的静态或时变的频率衰减。The dispersion caused by multipath propagation in free-space optical transmission systems and modal dispersion in multimode fibers can be relatively static or time-varying, and can lead to corresponding static or time-varying frequency attenuation.
在单模光纤中不存在多路径或模态色散,然而通常通过单模光纤传输的信号经历包括色度色散和偏振模色散(polarisation mode dispersion,PMD)的各种形式的劣化,根据所述色度色散,信号的不同频率分量以不同的速度传播,根据所述偏振模色散,耦合到不同偏振状态的信号的分量以不同速度传播。这两个色散机制以及其他包括各种非线性过程的机制,可导致所传输的脉冲的展宽(spreading)。There is no multipath or modal dispersion in single-mode fiber, however signals typically transmitted over single-mode fiber experience various forms of degradation including chromatic dispersion and polarization mode dispersion (PMD), according to which According to polarization mode dispersion, different frequency components of a signal propagate at different speeds, and according to which components of a signal coupled to different polarization states propagate at different speeds. These two dispersion mechanisms, as well as others including various nonlinear processes, can lead to spreading of the transmitted pulse.
此外,所有光传输通道将呈现用于发送和接收的电子和光电组件的特性所导致的其它形式的频率相关的失真。In addition, all optical transmission channels will exhibit other forms of frequency-dependent distortion caused by the characteristics of the electronic and optoelectronic components used for transmission and reception.
已知用于减轻或补偿通信通道中的频率相关的失真的方法,且已被广泛应用在射频(RF)通信系统中,包括无线和有线线路系统。用于减轻RF通信通道的有害影响的特别有效的方法包括将用于传输的信息编码成离散的数据块,然后该数据块通过通信通道被传输。在接收机处,每一个数据块被恢复,且优选地在频域执行均衡功能(equalisation function),以减少或消除在通道中引入的频率相关的影响。许多这样的方法使用某种形式的多载波调制,其中串行数据流被转换成包括一些并行数据流的块,然后该块在单独的频率的副载波上传输。在接收端,接收每个并行数据流,根据通道频率响应对其进行均衡,并解码该并行数据流以恢复被提供以进行传输的原始串行数据流。Methods for mitigating or compensating for frequency-dependent distortion in communication channels are known and have been widely used in radio frequency (RF) communication systems, including wireless and wireline systems. A particularly effective method for mitigating the detrimental effects of an RF communication channel involves encoding information for transmission into discrete data blocks, which are then transmitted over the communication channel. At the receiver, each data block is recovered and an equalization function is preferably performed in the frequency domain to reduce or eliminate frequency-dependent effects introduced in the channel. Many of these methods use some form of multicarrier modulation, in which a serial data stream is converted into a block comprising a number of parallel data streams, which are then transmitted on subcarriers at separate frequencies. On the receiving end, each parallel data stream is received, equalized according to the channel frequency response, and decoded to recover the original serial data stream that was presented for transmission.
更普遍的形式的多载波调制方法是正交频分复用(OFDM)。在OFDM系统中,每个副载波是正弦曲线,且选择副载波的特定集使得在每个传输的符号周期所有副载波相互正交。在特别便利的实现中,所需的正交性通过分别在发送机和接收机中使用正交变换来实现,且在OFDM的特定情况下使用离散傅立叶变换。A more common form of multicarrier modulation method is Orthogonal Frequency Division Multiplexing (OFDM). In an OFDM system, each subcarrier is sinusoidal, and a particular set of subcarriers is chosen such that all subcarriers are mutually orthogonal during each transmitted symbol period. In a particularly convenient implementation, the required orthogonality is achieved by using an orthogonal transform in the transmitter and receiver respectively, and in the particular case of OFDM using a discrete Fourier transform.
前述类型的调制方法,包括例如OFDM的多载波调制方案,与使用在高调制速率下调制的单个载波的传输相比提供了一些优点。这些优点包括在具有复杂时间展宽(complex time spreading)的色散通道上操作的能力、相位对频率和幅度对频率特性。因此,在例如自由空间通道、多模光纤和单模光纤的色散光通信通道上使用这种调制方法看起来是合乎期望的。Modulation methods of the aforementioned type, including multi-carrier modulation schemes such as OFDM, offer several advantages over transmission using a single carrier modulated at high modulation rates. These advantages include the ability to operate on dispersive channels with complex time spreading, phase versus frequency and amplitude versus frequency characteristics. Therefore, it seems desirable to use this modulation method on dispersive optical communication channels such as free space channels, multimode fiber and single mode fiber.
然而,这种调制方案在光通道上的使用到目前已被证明是有问题的。前面提到的在RF通信系统中已被证明非常有效的调制方法都包括生成和传输双极信号,该双极信号是具有幅度的正负偏移(excursion)的信号。双极信号在RF系统中容易生成和传输,在RF系统中载波是作为时变电压和电流生成和检测的电磁场,其可根据期望的信号幅度采取任意的正或负值。However, the use of such modulation schemes over optical channels has proven problematic so far. The aforementioned modulation methods that have proven to be very effective in RF communication systems all involve generating and transmitting bipolar signals, which are signals with positive and negative excursions in amplitude. Bipolar signals are easily generated and transmitted in RF systems, where the carrier is an electromagnetic field generated and detected as a time-varying voltage and current, which can take arbitrary positive or negative values depending on the desired signal amplitude.
然而,最简单且最容易实现的形式的光调制是强度调制(IntensityModulation,IM),其中所传输的信号的幅度由瞬时光功率表示。这本质上是个单极调制系统,因为功率或强度的负值在物理上没有意义,且不能生成。However, the simplest and easiest-to-implement form of optical modulation is intensity modulation (Intensity Modulation, IM), where the amplitude of the transmitted signal is represented by the instantaneous optical power. This is essentially a unipolar modulation system, since negative values of power or intensity have no physical meaning and cannot be generated.
在过去为克服单极强度调制所固有的限制所进行的努力已被证明大部分没有效果或不切实际。一个解决方案是在发送机处应用相对高的偏置水平,使得传输信号的幅度的正负偏移由在发送机的固定平均光输出功率附近的变化来表示。但是,这在其利用可用的光传输功率时非常低效,在多载波调制方案的情况下尤其低效,多载波调制方案可生成具有很大峰值平均功率比的信号,使得峰值偏移比信号平均值大很多。光功率低效在所有类型的光传输系统中可能都是一个问题,而在自由空间系统中的另外的问题是所需要的光输出功率可能超过眼睛安全水平(eye-safe level)。Efforts in the past to overcome the limitations inherent in unipolar intensity modulation have proven largely ineffective or impractical. One solution is to apply a relatively high bias level at the transmitter such that positive and negative excursions in the amplitude of the transmitted signal are represented by variations around a fixed average optical output power of the transmitter. However, this is very inefficient as it utilizes the available optical transmission power, and is especially inefficient in the case of multicarrier modulation schemes, which can generate signals with large peak-to-average power ratios such that the peak-to-average power ratio The average is much larger. Optical power inefficiency can be a problem in all types of optical delivery systems, and an additional problem in free-space systems is that the required optical output power can exceed eye-safe levels.
将双极信号编码到光通道上的其它方法包括使用更复杂的光调制方案,如具有差分调制(differential modulation)的多个光载波,或使用频率、相位或偏振调制(polarisation modulation)方法。然而所有这些方法都导致额外的成本和复杂性,而一些还需要使用相干光通道,因此这些方法对于许多相关应用来说一般太贵或者不切实际。因此期望提供一种方法和设备,该方法和设备采用合适的调制和均衡方案来补偿光通道特性,且特别地是在光通道中出现的各种类型的色散的影响,同时减轻已知的光调制方法在应用到双极信号时的上述缺点,而这也是本发明的目的。Other methods of encoding bipolar signals onto optical channels include using more complex optical modulation schemes, such as multiple optical carriers with differential modulation, or using frequency, phase, or polarization modulation methods. All of these approaches, however, incur additional cost and complexity, and some also require the use of coherent optical channels, so these approaches are generally too expensive or impractical for many relevant applications. It is therefore desirable to provide a method and apparatus that employs suitable modulation and equalization schemes to compensate for the effects of optical channel characteristics, and in particular the various types of dispersion that occur in optical channels, while at the same time mitigating the known optical The above-mentioned disadvantages of the modulation method when applied to bipolar signals, which is also the object of the present invention.
发明内容Contents of the invention
在一个方面,本发明提供了一种操作光发送机以通过色散的光通道发送数字信息的方法,所述方法包括如下步骤:In one aspect, the invention provides a method of operating an optical transmitter to transmit digital information over a dispersed optical channel, the method comprising the steps of:
将数字信息编码成多个数据块,各块包括许多信息位;encode digital information into blocks of data, each block comprising a number of bits of information;
生成与所述数据块的每个相对应的时变电信号;以及generating a time-varying electrical signal corresponding to each of said data blocks; and
将时变电信号应用于光发送机,以生成包括被调制到光载波上的不对称限幅(amplitude-limited)的传输信号的光信号,applying a time-varying electrical signal to an optical transmitter to generate an optical signal comprising an asymmetric amplitude-limited transmission signal modulated onto an optical carrier,
从而通过色散的光通道传输所传输的信息之后可通过使用包括接收信号的频域均衡的方法来恢复所传输的信息。The transmitted information can thus be recovered by using a method including frequency domain equalization of the received signal after transmission through the dispersed optical channel.
因此,本发明的方法包括传输光信号,该光信号可能是不对称限幅且更特别地可能是在零光输出功率水平被削波的强度调制的信号。从而该方法仅通过对于通过光通道传输允许所传输的信号的正或负幅度偏移被限制,而使得双极信号能够在单极光通道上传输。这减少或消除了在发送机处应用高的光偏置的需要,从而提高了光功率效率并使得输出功率水平能够保持在适用的安全水平之下。Thus, the method of the invention comprises transmitting an optical signal, which may be an asymmetrically clipped and more particularly may be an intensity modulated signal clipped at zero optical output power level. The method thus enables bipolar signals to be transmitted over a unipolar optical channel by only allowing positive or negative amplitude excursions of the transmitted signal to be limited for transmission over the optical channel. This reduces or eliminates the need to apply high optical bias at the transmitter, thereby improving optical power efficiency and enabling output power levels to remain below applicable safety levels.
在另一方面中,本发明提供了一种接收通过色散的光通道传输的数字信息的方法,其中数字信息被编码成多个数据块,各数据块包括许多信息位,且其中与数据块相对应的时变信号被调制到光载波上,以生成包括不对称限幅的传输信号的光信号,所述接收方法包括:In another aspect, the present invention provides a method of receiving digital information transmitted over a dispersive optical channel, wherein the digital information is encoded into a plurality of data blocks, each data block comprising a number of information bits, and wherein The corresponding time-varying signal is modulated onto an optical carrier to generate an optical signal comprising an asymmetrically clipped transmission signal, the receiving method comprising:
检测光信号以生成与不对称限幅的传输信号相对应的所接收的电信号;detecting the optical signal to generate a received electrical signal corresponding to the asymmetrically clipped transmitted signal;
对所接收的电信号进行频域均衡,以减轻光通道的色散对所传输的光信号的影响;以及performing frequency-domain equalization on the received electrical signal to mitigate the effects of dispersion of the optical channel on the transmitted optical signal; and
解码均衡的信号以恢复编码的数据块和相对应的所传输的数字信息。The equalized signal is decoded to recover the encoded data blocks and corresponding transmitted digital information.
在另一方面中,本发明提供了一种通过色散的光通道传送数字信息的方法,该方法包括如下步骤:In another aspect, the present invention provides a method of transmitting digital information through a dispersed optical channel, the method comprising the steps of:
将数字信息编码成多个数据块,各数据块包括许多信息位;encoding digital information into a plurality of data blocks, each data block comprising a number of bits of information;
生成与所述数据块的每一个相对应的时变电信号;generating a time-varying electrical signal corresponding to each of said data blocks;
将时变电信号应用于光发送机,以生成包括被调制到光载波上的不对称限幅的传输信号的光信号;applying a time-varying electrical signal to an optical transmitter to generate an optical signal comprising an asymmetrically clipped transmission signal modulated onto an optical carrier;
通过色散的光通道传输光信号;Transmission of optical signals through dispersive optical channels;
检测光信号以生成与不对称限幅的传输信号相对应的所接收的电信号;detecting the optical signal to generate a received electrical signal corresponding to the asymmetrically clipped transmitted signal;
对所接收的电信号进行频域均衡,以减轻光通道的色散对所传输的光信号的影响;以及performing frequency-domain equalization on the received electrical signal to mitigate the effects of dispersion of the optical channel on the transmitted optical signal; and
解码均衡的信号以恢复编码的数据块和相对应的所传输的数字信息。The equalized signal is decoded to recover the encoded data blocks and corresponding transmitted digital information.
编码的步骤优选地包括生成与信息位相对应的多个符号值。每个所述符号值可编码一个或多个信息位,因此符号值的数量可能等于或小于在每个数据块内被编码的信息位的数量。The step of encoding preferably includes generating a plurality of symbol values corresponding to the information bits. Each of said symbol values may encode one or more information bits, so the number of symbol values may be equal to or less than the number of information bits encoded within each data block.
符号值可以是例如根据信息位和符号值之间的所期望的映射而生成的实数或复数值。适当的映射方法可与传统的信号调制格式相对应,且可包括但不限于开关键控(On-Off keying,OOK)、幅移键控(Amplitude ShiftKeying,ASK)、正交幅度调制(Quadrature Amplitude Modulation,QAM)、相移键控(Phase Shift Keying,PSK)、频移键控(Frequency ShiftKeying,FSK)等。The symbol value may be, for example, a real or complex value generated according to a desired mapping between information bits and symbol values. Appropriate mapping methods may correspond to traditional signal modulation formats and may include, but are not limited to, On-Off Keying (OOK), Amplitude Shift Keying (ASK), Quadrature Amplitude Modulation (Quadrature Amplitude Modulation, QAM), Phase Shift Keying (Phase Shift Keying, PSK), Frequency Shift Keying (Frequency Shift Keying, FSK), etc.
在本发明的一个优选实施例中,编码和生成步骤根据频域中的多载波方法进行,且在一个有利的实现中根据OFDM方法进行。In a preferred embodiment of the invention, the coding and generating steps are carried out according to a multicarrier method in the frequency domain, and in an advantageous implementation according to an OFDM method.
因此,生成时变电信号的步骤可包括计算每个数据块的数字符号值的离散傅立叶逆变换(IDFT)。有利地,可使用快速傅立叶变换(FFT)算法来计算IDFT。本领域的技术人员将理解,在其它实施例中可使用基于例如正交基函数和相对应的变换的可替代集的到时域的可替代变换。Accordingly, the step of generating a time-varying electrical signal may comprise computing an inverse discrete Fourier transform (IDFT) of the digital symbol values of each data block. Advantageously, the IDFT may be calculated using a Fast Fourier Transform (FFT) algorithm. Those skilled in the art will appreciate that in other embodiments alternative transformations to the time domain based on alternative sets such as orthogonal basis functions and corresponding transformations may be used.
优选地,将从逆变换生成的离散数字值从并行转换成串行格式,且使用数模转换来生成实数的时变电信号。Preferably, the discrete digital values generated from the inverse transform are converted from parallel to serial format, and digital-to-analog conversion is used to generate real time-varying electrical signals.
生成步骤还可包括在数模转换之前添加防护频带或循环前缀,这有利地有助于接收机处的均衡处理。The generating step may also include adding guard bands or cyclic prefixes prior to digital-to-analog conversion, which advantageously facilitates equalization processing at the receiver.
可使用许多不同的方法来实现不对称限幅的传输的光信号的生成。例如,在一个布置中,使用导致在零光输出功率水平所传输的信号的削波(clipping)的减小的偏置水平直接调制发光二极管(LED)或半导体激光器二极管。适当的偏置水平可小于电信号的峰-峰幅度的50%,且在一些实施例中甚至有可能应用有效的负偏置,使得除了完全抑制负偏移之外,信号的正偏移也在某种程度上得到抑制。The generation of an asymmetrically clipped transmitted optical signal can be achieved using many different methods. For example, in one arrangement a light emitting diode (LED) or semiconductor laser diode is directly modulated with a reduced bias level resulting in clipping of the transmitted signal at zero optical output power level. A suitable bias level may be less than 50% of the peak-to-peak amplitude of the electrical signal, and in some embodiments it is even possible to apply an effective negative bias such that in addition to completely suppressing negative excursions, positive excursions of the signal are also suppressed. suppressed to some extent.
生成限幅的传输的光信号的可替代方法包括在调制之前在数字或模拟(电学)域中进行限制、偏置和/或其它幅度调节,以消除或减少对光发送机的固有限制(limiting)的依赖。例如,除了提供偏置调节之外,电信号可在调制到光载波上之前被放大、衰减和/或逆变(invert),且应注意逆变的应用可导致所传输的信号的正偏移,而不是负偏移的幅度的有效限制。在数模转换之前在数字域中进行该操作特别地有益,以最有效地使用转换硬件的转换范围和分辨率。Alternative methods of generating a limited transmitted optical signal include limiting, biasing and/or other amplitude adjustments in the digital or analog (electrical) domain prior to modulation to eliminate or reduce the limiting limitations inherent in optical transmitters. ) dependencies. For example, in addition to providing bias adjustment, electrical signals may be amplified, attenuated, and/or inverted prior to being modulated onto an optical carrier, and it should be noted that application of inversion may result in a positive offset to the transmitted signal , not an effective limit on the magnitude of negative offsets. It is particularly beneficial to do this in the digital domain prior to digital-to-analog conversion to make the most efficient use of the conversion range and resolution of the conversion hardware.
本发明的发明人已经确定应用零偏置水平特别有利,这会导致信号的负偏移基本上完全被抑制,且特别地发明人已示出该布置与更高的偏置水平相比能够提供改进的光功率分配(optical power budget)。特别地,与幅度限制基本上被避免的传统布置相比,有可能实现在功率分配上的显著改进。此外,至少在一些情况下,与如采用非归零(NRZ)或归零(RZ)波形的OOK IM信号的普通串行传输格式相比,可实现光功率分配的改进。The inventors of the present invention have determined that it is particularly advantageous to apply a zero bias level, which results in substantially complete suppression of negative excursions of the signal, and in particular the inventors have shown that this arrangement can provide Improved optical power budget. In particular, it is possible to achieve a significant improvement in power distribution compared to conventional arrangements where amplitude limitations are substantially avoided. Furthermore, improvements in optical power distribution can be achieved, at least in some cases, compared to common serial transmission formats such as OOK IM signals employing non-return-to-zero (NRZ) or return-to-zero (RZ) waveforms.
调制具有不对称限幅的传输信号的光载波的其它方法可包括结合如Mach-Zehnder或电吸收型调制器的单独或集成的外部调制器使用如激光器或LED的连续波(continuous wave,CW)光源。本领域技术人员将理解大量适当的光载波调制方法是可用的。Other methods of modulating the optical carrier of the transmitted signal with asymmetric clipping may include the use of continuous wave (CW) such as lasers or LEDs in conjunction with separate or integrated external modulators such as Mach-Zehnder or electroabsorption type modulators. light source. Those skilled in the art will appreciate that a number of suitable optical carrier modulation methods are available.
传输光信号的步骤可包括通过自由空间、通过多模光纤或通过单模光纤或可替换地通过任何其它适当的导向性(guided)或非导向性(unguided)光介质进行传输。The step of transmitting the optical signal may comprise transmitting through free space, through multimode fiber or through single mode fiber or alternatively through any other suitable guided or unguided optical medium.
检测光信号的步骤优选地包括例如使用如光电二极管、雪崩光电二极管(APD)等的检测器-如需要与电放大器一起-进行光电转换。The step of detecting the optical signal preferably includes photoelectric conversion, for example using a detector such as a photodiode, avalanche photodiode (APD) or the like - if necessary together with an electrical amplifier.
进行电信号的频域均衡的步骤优选地包括采样信号以产生离散数字值的序列。然后可进行与在生成时变电信号时进行的逆变换相对应的从时域的变换以生成相对应的多个所接收的符号值。在一个优选的实施例中,有利地使用FFT算法进行所采样的信号的离散傅立叶变换(DFT),以计算所接收的符号值。均衡然后优选地包括进行每个所接收的符号值的幅度和/或相位调节,以基本上均衡光通道的色散对所传输的光信号的影响,使得所接收的被均衡的符号值更近似地接近所传输的符号值。The step of performing frequency domain equalization of the electrical signal preferably comprises sampling the signal to produce a sequence of discrete digital values. A transformation from the time domain corresponding to the inverse transformation performed when generating the time-varying electrical signal may then be performed to generate a corresponding plurality of received symbol values. In a preferred embodiment, a Discrete Fourier Transform (DFT) of the sampled signal is advantageously performed using an FFT algorithm to calculate the received symbol values. Equalization then preferably includes performing an amplitude and/or phase adjustment of each received symbol value to substantially equalize the effect of the dispersion of the optical channel on the transmitted optical signal such that the received equalized symbol values are more approximately Close to the transmitted symbol value.
解码均衡的信号的步骤优选地包括从均衡的所接收符号值恢复原始信息位。特别优选的是使用与在编码步骤中应用的映射相对应的解映射方法来恢复原始信息位。如上所述,适当的映射和解映射方法对应于传统的信号调制格式,包括OOK、ASK、QAM、PSK、FSK等。The step of decoding the equalized signal preferably comprises recovering the original information bits from the equalized received symbol values. It is particularly preferred to recover the original information bits using a demapping method corresponding to the mapping applied in the encoding step. As mentioned above, appropriate mapping and demapping methods correspond to conventional signal modulation formats, including OOK, ASK, QAM, PSK, FSK, etc.
在另一方面,本发明提供了一种用于通过色散的光通道传输数字信息的设备,所述设备包括:In another aspect, the present invention provides an apparatus for transmitting digital information over a dispersed optical channel, the apparatus comprising:
编码器,用于将数字信息编码成多个数据块,各数据块包括许多信息位;an encoder for encoding digital information into a plurality of data blocks, each data block comprising a number of bits of information;
信号发生器,用于生成与所述数据块的每个相对应的时变电信号;以及a signal generator for generating a time-varying electrical signal corresponding to each of said data blocks; and
光发送机,包括用于以时变电信号调制光载波,以生成包括被调制到光载波上的不对称限幅的传输信号的光信号的装置,an optical transmitter comprising means for modulating an optical carrier with a time-varying electrical signal to generate an optical signal comprising an asymmetrically clipped transmission signal modulated onto the optical carrier,
从而所传输的信息可在通过色散的光通道传输之后通过相对应的接收机被恢复,所述接收机包括用于进行所接收信号的频域均衡的装置。The transmitted information can thus be recovered after transmission through the dispersive optical channel by a corresponding receiver comprising means for frequency-domain equalization of the received signal.
在优选实施例中,编码器包括用于接收信息位和生成相对应的多个符号值的多个映射单元。映射单元可实现许多适当的映射方法中的任何一个或多个,所述映射方法包括与如OOK、ASK、QAM、PSK、FSK等的传统调制格式相对应的映射。在优选实施例中,如在数字信号处理领域众所周知的那样,映射单元可使用数字硬件和/或软件装置来实现。In a preferred embodiment, the encoder comprises a plurality of mapping units for receiving information bits and generating a corresponding plurality of symbol values. The mapping unit may implement any one or more of a number of suitable mapping methods, including mappings corresponding to conventional modulation formats such as OOK, ASK, QAM, PSK, FSK, and the like. In a preferred embodiment, the mapping unit is implemented using digital hardware and/or software means, as is well known in the field of digital signal processing.
信号发生器可包括用于实施逆变换以生成数字时域信号的数字硬件和/或软件装置。在特别优选的实施例中,信号发生器包括用于优选地使用FFT算法计算IDFT的装置。信号发生器还可包括用于根据变换发生器(transform generator)的输出生成数字样本的时间序列的并-串转换器,以及用于生成连续时变电信号的数模转换器(DAC)。在优选实施例中,信号发生器被配置成将防护时间和/或循环前缀插入到时变信号中,以帮助在相对应的接收机处在频域中对所接收信号的均衡。A signal generator may include digital hardware and/or software means for performing an inverse transform to generate a digital time domain signal. In a particularly preferred embodiment, the signal generator comprises means for calculating the IDFT, preferably using an FFT algorithm. The signal generator may also include a parallel-to-serial converter for generating a time-series of digital samples from the output of the transform generator, and a digital-to-analog converter (DAC) for generating a continuous time-varying electrical signal. In a preferred embodiment, the signal generator is configured to insert a guard time and/or a cyclic prefix into the time-varying signal to facilitate equalization of the received signal in the frequency domain at a corresponding receiver.
光发送机优选地包括如LED、半导体激光器或其它光辐射源的光源。光发送机还可包括用于直接调制光源的电驱动电路,且可替代地或另外地可包括外部光调制器,如Mach-Zehnder或电吸收型调制器等。The optical transmitter preferably comprises a light source such as an LED, semiconductor laser or other source of optical radiation. The optical transmitter may also include electrical drive circuitry for directly modulating the light source, and may alternatively or additionally include an external optical modulator, such as a Mach-Zehnder or electro-absorption type modulator, or the like.
在又一方面中,本发明提供了一种用于接收通过色散的光通道传输的数字信息的设备,其中数字信息被编码成多个数据块,各数据块包括许多信息位,且其中与数据块相对应的时变信号被调制到光载波上以生成包括不对称限幅的传输信号的光信号,所述接收设备包括:In yet another aspect, the present invention provides an apparatus for receiving digital information transmitted over a dispersive optical channel, wherein the digital information is encoded into a plurality of data blocks, each data block comprising a number of information bits, and wherein the The time-varying signal corresponding to the block is modulated onto an optical carrier to generate an optical signal including an asymmetrically clipped transmission signal, and the receiving device includes:
光检测器,用于检测光信号以产生与不对称限幅的传输信号相对应的所接收的电信号;an optical detector for detecting the optical signal to generate a received electrical signal corresponding to the asymmetrically clipped transmitted signal;
用于进行所接收的电信号的频域均衡以减轻光通道的色散对所传输的光信号的影响的装置;以及means for performing frequency domain equalization of received electrical signals to mitigate the effects of dispersion of the optical channel on transmitted optical signals; and
解码器,用于解码均衡的信号以恢复被编码的数据位和相对应的所传输的数字信息。A decoder for decoding the equalized signal to recover the encoded data bits and corresponding transmitted digital information.
在优选实施例中,光检测器包括用于光信号的光电转换的适当的器件,如光电二极管、雪崩光电二极管(APD)等。光检测器还可包括用于调节信号水平和调整接收信号的电子器件,如放大器、滤波器等。In a preferred embodiment, the photodetector comprises a suitable device for photoelectric conversion of the light signal, such as a photodiode, an avalanche photodiode (APD), or the like. Photodetectors may also include electronics, such as amplifiers, filters, etc., for adjusting signal levels and conditioning received signals.
频域均衡装置优选地包括用于采样被检测的信号并生成表示该信号的数字化时间序列的模数转换器(ADC)。The frequency domain equalization means preferably includes an analog-to-digital converter (ADC) for sampling the detected signal and generating a digitized time series representative of the signal.
频域均衡装置优选地还包括用于进行采样的数据的串-并转换的数字硬件和/或软件装置。另外,在优选实施例中,频域均衡装置包括用于计算数字化信号到频域的变换的硬件和/或软件装置,更优选的是用于例如使用FFT算法进行DFT的装置。根据优选实施例,所述变换的输出是多个接收的符号值。The frequency domain equalization means preferably also comprise digital hardware and/or software means for serial-to-parallel conversion of the sampled data. Furthermore, in a preferred embodiment the frequency domain equalization means comprise hardware and/or software means for computing the transformation of the digitized signal into the frequency domain, more preferably means for performing a DFT eg using an FFT algorithm. According to a preferred embodiment, the output of said transformation is a plurality of received symbol values.
频域均衡装置优选地还包括均衡器库,该均衡器库包括用于调节每个所接收的符号值的幅度和/或相位以基本上均衡色散的光通道对所传输的信号的影响的多个滤波器。The frequency-domain equalization means preferably further comprises an equalizer bank comprising a multiplier for adjusting the magnitude and/or phase of each received symbol value to substantially equalize the effect of the dispersed optical channel on the transmitted signal. filter.
根据本发明的优选实施例,解码器包括用于接收均衡的符号值并生成相对应的数字信息位的多个解映射单元。如将了解的,解映射单元可与在相对应的发送机中采用的映射单元相对应,且因此可实现任何合适的解映射方法,如与包括OOK、ASK、QAM、PSK、FSK等传统调制格式相对应的解映射。在优选实施例中,如在数字信号处理领域中众所周知的那样,解映射单元可使用数字硬件和/或软件装置来实现。According to a preferred embodiment of the invention, the decoder comprises a plurality of demapping units for receiving equalized symbol values and generating corresponding digital information bits. As will be appreciated, the demapping unit may correspond to the mapping unit employed in the corresponding transmitter, and thus any suitable demapping method may be implemented, such as conventional modulation including OOK, ASK, QAM, PSK, FSK, etc. format corresponding to the demap. In a preferred embodiment, the demapping unit may be implemented using digital hardware and/or software means, as is well known in the field of digital signal processing.
在另一方面中,还提供了包括通过一个或多个色散的光通道与根据本发明的一个或多个接收设备通信的根据本发明的一个或多个发送设备的系统。In another aspect, there is also provided a system comprising one or more transmitting devices according to the invention communicating with one or more receiving devices according to the invention via one or more dispersed optical channels.
通过以下对本发明的优选实施例的说明,本发明的其它优选特征和优点对于本领域的技术人员将更明显,所述优选实施例不应该被认为是对在前述说明或在所附权利要求中限定的本发明的范围的限制。Other preferred features and advantages of the present invention will be apparent to those skilled in the art from the following description of preferred embodiments of the present invention, which should not be construed as limiting in the preceding description or in the appended claims. limit the scope of the invention.
附图说明Description of drawings
参照附图说明本发明的优选实施例,在附图中:Preferred embodiment of the present invention is described with reference to accompanying drawing, in accompanying drawing:
图1示意性示出了根据本发明实施例在光通道上传送数字信息的系统;Fig. 1 schematically shows a system for transmitting digital information on an optical channel according to an embodiment of the present invention;
图2示出了在图1所示的系统的发送机内生成的双极OFDM波形;Figure 2 shows a bipolar OFDM waveform generated within the transmitter of the system shown in Figure 1;
图3示出了在图1的光通道上传输的单极OFDM波形;Figure 3 shows a unipolar OFDM waveform transmitted over the optical channel of Figure 1;
图4A是示出根据本发明实施例作为光偏置水平的函数的传输性能的图;Figure 4A is a graph showing transmission performance as a function of optical bias level according to an embodiment of the present invention;
图4B和4C示出了与在图4A中所示的两个不同的光偏置水平相对应的示例星座图;Figures 4B and 4C show example constellation diagrams corresponding to the two different light bias levels shown in Figure 4A;
图5是示出根据本发明实施例作为链路损耗的函数的接收信号质量的图;5 is a graph showing received signal quality as a function of link loss according to an embodiment of the invention;
图6示意性示出了根据本发明实施例用于在仅使用奇数编号的OFDM通道的光通道上传送数字信息的可替换系统;以及Figure 6 schematically illustrates an alternative system for transmitting digital information on optical channels using only odd-numbered OFDM channels according to an embodiment of the present invention; and
图7示出了根据本发明的可替换实施例通过将所传输的信号映射到副载波对上所生成的波形。Figure 7 shows a waveform generated by mapping the transmitted signal onto subcarrier pairs according to an alternative embodiment of the present invention.
具体实施方式Detailed ways
首先看图1,图1示意性示出了一种根据本发明优选实施例在光通道上传送数字信息的系统。虽然本发明在此以在编码和调制信号以在光通道上传输时采用正交频分复用(OFDM)的系统100为例,应理解本发明不限于该特定实施例。更准确地说,在本发明的实施例中,一般地以块编码数字信息,在发送机处可选地添加防护时间或循环前缀以及通过光通道传输接收信号之后在频域均衡接收信号来表征。虽然OFDM提供了用于实现这些功能的一种有利的方式,但是通过参照OFDM实现对优选实施例的说明将被理解为仅是示例性的,并不限制在前述发明内容和所附权利要求中更广泛限定的本发明。Referring first to FIG. 1, FIG. 1 schematically shows a system for transmitting digital information over an optical channel according to a preferred embodiment of the present invention. Although the invention is illustrated herein with respect to a
示例性系统100包括发送设备102和接收设备104,为方便在此分别更一般地将其称作“发送机”和“接收机”。
发送机102通过色散的光通道106与接收机104通信。虽然在图1中将光通道106示意性地表示为圈状光纤(coiled optical fibre),例如多模光纤,应理解光通道106可包括单模或多模光纤、自由空间链路或任何其它适当的导向或非导向光介质。The transmitter 102 communicates with the receiver 104 via a dispersive optical channel 106 . Although optical channel 106 is shown schematically in FIG. 1 as a coiled optical fiber, such as a multimode fiber, it should be understood that optical channel 106 may comprise single-mode or multimode fiber, a free-space link, or any other suitable optical fiber. guided or unguided optical media.
发送机102包括映射单元110、信号发生器114和光发送机122,以下将详细说明其细节和操作。接收机104包括光检测器124、包括信号解码组件128、130、132和均衡器库136的频域均衡器、以及解映射单元140。同样以下提供对接收机104的细节及其操作的说明。The transmitter 102 includes a mapping unit 110, a signal generator 114 and an optical transmitter 122, the details and operation of which will be described in detail below. The receiver 104 includes a photodetector 124 , a frequency domain equalizer including signal decoding components 128 , 130 , 132 and an equalizer bank 136 , and a demapping unit 140 . A description of the details of receiver 104 and its operation is also provided below.
用于在光通道106上传输的数字信息通过并行输入端口108被输入到发送机102。输入的数字信息以块接收,每个块包括与输入端口108中的并行输入的数量相对应的许多信息位。应理解可替换地可提供数字信息的其它形式的输入,如串行输入端口。包括在每个块中的信息位的数量一般被预定,且在任何特定实施例中可以是固定数量的位,或者可随时间改变。例如,每块的位的数量可根据如测量的通道质量、所期望的信息比特率和/或其它系统参数的各种因素而被动态预定。Digital information for transmission over optical channel 106 is input to transmitter 102 through parallel input port 108 . Incoming digital information is received in blocks, each block comprising a number of bits of information corresponding to the number of parallel inputs in input port 108 . It will be appreciated that other forms of input of digital information may alternatively be provided, such as a serial input port. The number of information bits included in each block is generally predetermined, and may be a fixed number of bits in any particular embodiment, or may vary over time. For example, the number of bits per block may be dynamically predetermined based on various factors such as measured channel quality, desired information bit rate, and/or other system parameters.
包括多个输入的位的每个块被编码以生成多个符号值,其在示例性发送机102中沿着并行信号路径112传送。根据系统100的优选实施例,映射单元110使用QAM映射来编码输入的数据位以提供得到的编码的符号值。每个QAM信号值是表示要应用于所传输的信号的相对应的频率载波的幅度和相位调制的复数。然而应理解可使用可替代的映射方案来编码输入的数据,包括但不限于OOK、ASK、PSK、FSK等。Each block comprising multiple input bits is encoded to generate multiple symbol values, which are transmitted along parallel signal paths 112 in the exemplary transmitter 102 . According to a preferred embodiment of
发送机102中的信号发生器114将在并行信号线112上提供的每个数据块的符号值转换成相对应的连续时变电信号。根据系统100的示例性实施例,且根据OFDM方法,信号发生器114包括频率-时间变换116、并-串转换器118和数模转换器(DAC)120。使用离散傅立叶逆变换(IDFT)实现频率-时间变换116,其可如本领域众所周知的那样使用数字电子硬件或软件装置或者硬件和软件的组合而容易地提供。Signal generator 114 in transmitter 102 converts the symbol value of each data block provided on parallel signal line 112 into a corresponding continuous time-varying electrical signal. According to an exemplary embodiment of the
在示例性实施例100中,IDFT 116产生并行数据输出,其被并-串转换器118转换成所需的时变电信号波形的数字化样本序列。然后使用DAC120来生成连续时变的电信号。In the
通过电光(E/O)转换器122将时变电信号转换成光形式。E/O转换器122是光发送机,用于生成包括被调制到光载波上的不对称限幅的传输信号的光信号。特别地,从光发送机122输出的光信号是光发送机122对从DAC 120输出的时变电信号适当调制后产生的强度调制的信号。The time-varying electrical signal is converted to optical form by an electro-optic (E/O) converter 122 . E/O converter 122 is an optical transmitter for generating an optical signal comprising an asymmetrically clipped transmission signal modulated onto an optical carrier. In particular, the optical signal output from the optical transmitter 122 is an intensity-modulated signal generated after the optical transmitter 122 properly modulates the time-varying electrical signal output from the DAC 120.
应理解IDFT 116的输出一般是复数值,而需要实数值的波形来调制光发送机122。在本领域中已知各种用于提供所需的实数值信号的方法,且在示例性实施例100中使用向上转换(upconversion)方法,其中复数值的实部和虚部在电学域中被正交调制到RF副载波上。所需操作可例如或者数字地(即在DAC 120之前)进行,或者使用两个DAC 120来对实部和虚部分别进行,随后是适当的模拟电子装置。由于所需的组件和操作对于本领域技术人员来说是明显的,所以在图1的示意图中将其省略。It should be understood that the output of IDFT 116 is generally complex-valued, whereas a real-valued waveform is required to modulate optical transmitter 122. Various methods are known in the art for providing the required real-valued signal, and in
通过图2和3所示的波形示出了将时变电信号应用到光发送机122以生成包括被调制到光载波上的不对称限幅的传输信号的光信号。图2示出在DAC 120的输出处生成的示例性OFDM波形200。显然OFDM波形200是双极波形,包括正幅度偏移(例如202)和负幅度偏移(例如204)。之前一般认为如波形200的双极信号对使用具有直接检测的强度调制(intensity modulation with direct detection,IM/DD)的光传输系统提出了特定的困难。特别地,由于光发送机的输出功率或强度必须总是为零或为正,所以不可能在IM/DD光通道上直接传输具有零平均值的双极波形,如波形200。因此常见的方法是对光发送机应用偏置水平以偏移平均输出功率,从而容纳负信号偏移,例如204。然而该方法浪费光功率分配,且可能需要相对高的光输出功率水平,从而产生潜在的安全问题,尤其是在自由空间光链路上。The application of a time-varying electrical signal to the optical transmitter 122 to generate an optical signal comprising an asymmetrically clipped transmission signal modulated onto an optical carrier is illustrated by the waveforms shown in FIGS. 2 and 3 . FIG. 2 shows an
因此,根据本发明,在光发送机122内应用非常低或甚至为零的偏置水平,以产生光输出信号,该光输出信号由于在零输出功率水平的负偏移(例如204)的削波(clipping)而不对称限幅。图3示出了根据本发明生成的示例性光输出功率波形300,其中在发送机122中不应用光偏置。波形300包括正功率偏移(例如302),然而负偏移基本上被抑制在零输出功率水平304。Thus, in accordance with the present invention, a very low or even zero bias level is applied within the optical transmitter 122 to produce an optical output signal that is clipped due to a negative offset (e.g., 204) at zero output power level. Wave (clipping) asymmetric clipping. FIG. 3 illustrates an exemplary optical
在光发送机122内可采用多种不同的布置来从如由OFDM波形200表示的双极时变电信号生成输出光波形300。例如,LED或半导体激光器可直接在零偏置水平调制或者使用小于电信号200的峰-峰幅度的50%的减小的偏置水平来调制。实际上,在一些实施例中,甚至可应用有效的负偏置。对传输信号的所需的偏置和/或限制可通过光源和相关联的驱动电路进行,或可在如LED或半导体激光器的光源的调制之前在数字或模拟(电学)域中应用。例如如果由于波形中大的负偏移(例如204),双极波形(例如200)直接应用到光源可导致毁坏和/或如开启延迟的不期望的效果,那么该数字或模拟限制是合适的。实际上,为了避免在直接调制的光源中所谓的“存储效应”,例如打开延迟,结合足以避免光源的关闭和/或饱和的正偏置来提供负信号偏移的预调制限制是合乎期望的,也落在本发明的范围内。A number of different arrangements may be employed within optical transmitter 122 to generate output
可能特别有利的是,在数模转换之前在数字域进行一些或所有这样的信号处理,以最有效地使用DAC 120的转换范围和分辨率。It may be particularly advantageous to perform some or all of this signal processing in the digital domain prior to digital-to-analog conversion to make the most efficient use of the conversion range and resolution of DAC 120.
输出光波形300可替换地可通过结合例如使用Mach-Zehnder或电吸收型调制器的光源的外部调制,使用合适的数字和/或模拟限制、偏置或其它幅度调节来生成。对于本领域技术人员来说显然应理解用于生成光输出波形300的所有这种方法都落在本发明的范围内。The
现在返回图1,不对称限幅的光信号通过光通道106传输,并使用光电(optical to electrical,O/E)转换器124在接收机104处被接收。O/E转换器124是一般包括适当的光电二极管或等同器件、以及如放大器、滤波器等的相关联的电子电路的光检测器,且用于生成具有适当特性的所接收的电信号以在接收机104内进一步处理。Returning now to FIG. 1 , the asymmetrically clipped optical signal is transmitted through optical channel 106 and received at receiver 104 using optical to electrical (O/E) converter 124 . The O/E converter 124 is a photodetector that typically includes a suitable photodiode or equivalent, and associated electronic circuitry such as an amplifier, filter, etc., and is used to generate a received electrical signal with suitable characteristics to Further processing within the receiver 104 .
根据接收机104的示例性实施例,使用模数转换器(ADC)128将与不对称限幅的传输的光信号相对应的、得到的接收的电信号转换成相对应的数字化样本的序列。数字化样本的串行序列在串-并转换器130中被转换成并行形式,且样本的并行块通过离散傅立叶变换器(DFT)132被转换成沿着并行信号路径134输出的相对应的频域值集。应注意在系统100的示例性实施例中,优选地使用FFT算法实现的DFT 132是与在发送机102内进行的IFFT 116相对应的逆变换。According to an exemplary embodiment of receiver 104, an analog-to-digital converter (ADC) 128 is used to convert the resulting received electrical signal corresponding to the asymmetrically clipped transmitted optical signal into a corresponding sequence of digitized samples. The serial sequence of digitized samples is converted to parallel form in a serial-to-parallel converter 130, and the parallel block of samples is converted by a discrete Fourier transformer (DFT) 132 into a corresponding frequency-domain output along a parallel signal path 134. value set. It should be noted that in the exemplary embodiment of
从DFT 132输出的频域值包括与输入到发送机102中的IFFT 116的传输的符号值相对应的一组调制后的符号值。然而所接收的符号值已被通过色散的光通道106的传输所影响,因而在恢复原始的传输的数字信息中可出现误差。为了补偿光通道106对调制后的符号值的影响,使用均衡器库136应用均衡。在接收机104的示例性实施例中,均衡器库包括多个滤波器。在最简单的情况中,每个滤波器是复数乘法器(complex multiplier),该复数乘法器调节从DFT 132输出的相对应的所接收的符号值的幅度和/或相位以基本上均衡色散的光通道106对所传输的信号的影响。应理解该均衡可另外地补偿发送机102和接收机104的各种电子和光电组件的频率相关的非理想特性。The frequency domain values output from DFT 132 include a set of modulated symbol values corresponding to the transmitted symbol values input to IFFT 116 in transmitter 102. However, the received symbol values have been affected by transmission through the dispersed optical channel 106, so errors may occur in recovering the original transmitted digital information. To compensate for the effect of optical channel 106 on the modulated symbol values, equalization is applied using equalizer library 136 . In an exemplary embodiment of receiver 104, the equalizer bank includes a plurality of filters. In the simplest case, each filter is a complex multiplier that adjusts the magnitude and/or phase of the corresponding received symbol value output from the DFT 132 to substantially equalize the The effect of the optical channel 106 on the transmitted signal. It should be understood that the equalization may additionally compensate for frequency-dependent non-idealities of the various electronic and optoelectronic components of the transmitter 102 and receiver 104 .
得到的均衡的符号值沿着并行信号线138输出到与映射单元110相对应的解映射单元140。解映射单元140接收均衡的符号值,并生成相对应的数字信息位。得到的解码的信息沿着并行信号线142输出,且如果系统100内的噪声和/或失真的水平不是过高,那么信号线142上的输出的数字信息位一般与在信号线108输入的原始数字信息位匹配。当然本领域的技术人员应理解,由于通信系统中的噪声和/或失真可能会引入误差,因此所传输的信息位中的零误差率可能无法实现。但是,还应理解可采用另外的装置,包括插入在数字信息内的误差检测和误差校正码,以检测和/或校正该位误差。The resulting equalized symbol values are output to a demapping unit 140 corresponding to the mapping unit 110 along the parallel signal line 138 . Demapping unit 140 receives the equalized symbol values and generates corresponding digital information bits. The resulting decoded information is output along parallel signal line 142, and if the level of noise and/or distortion within
从上面的讨论应理解,ADC 128、串-并转换器130、DFT 132和均衡器库136组合在一起提供用于进行接收信号的频域均衡以减轻光通道106的色散对于所传输的光信号的影响的装置。而且,解映射单元140的功能是解码均衡的信号以恢复被编码的传输的数据块和相对应的传输的数字信息。虽然采用OFDM技术,包括使用离散傅立叶变换来提供本发明示例性实施例100内的这些功能,但是应理解在可替换的实施例中可提供用于提供频域均衡和用于解码均衡的信号的其它方式。It should be understood from the above discussion that ADC 128, serial-to-parallel converter 130, DFT 132, and equalizer bank 136 combine to provide frequency domain equalization of the received signal to mitigate dispersion of optical channel 106 for the transmitted optical signal device of influence. Furthermore, the function of the demapping unit 140 is to decode the equalized signal to recover the encoded transmitted data blocks and corresponding transmitted digital information. While OFDM techniques, including the use of discrete Fourier transforms, are employed to provide these functions within the
图4A示出图400,图400示出了作为在光发送机122中应用的光偏置水平的函数的系统传输性能的量度(measure)。在图4中示出的性能的特定量度是平均信号水平的平方与信号变化的比(即识别出具体的所传输的信号值的容易度的量度)除以平均光功率。根据传统方法,将应用范围402内的相对高的光偏置水平,以避免所传输的信号的负偏移的削波或限制。由于该传统方法需要更高水平的光功率的传输而没有任何信息传输上相对应的增加,所以根据图400中所示的量度其对应于相对低的性能。作为对比,当在发送机122中应用零偏置404时,平均的传输光功率被显著减小,从而尽管不对称幅度限制对传输信号有影响,也提供了在根据该量度的性能上的总的提高。FIG. 4A shows a
等同地,对于给定的发送光功率和总的光链路损耗,与偏置水平404相对应的接收信号质量优于与在范围402中的传统偏置水平相对应的信号质量。这在图4B和4C中得以示出,图4B和4C示出了在具有30dB的损耗的光链路上使用相同的平均源功率传输之后根据4-QAM方案映射的所接收的符号的示例性星座图。星座406对应于零偏置水平404,而星座408对应于范围402内的偏置水平。显然,由于接收机噪声对星座408所表示的所接收符号的相对大的影响,由星座406表示的所接收符号的质量优于由星座408表示的所接收符号的质量。Equivalently, for a given transmitted optical power and total optical link loss, the received signal quality corresponding to the
图5示出了相对应的曲线图500,曲线图500示出了作为光通道106的总损耗的函数的接收信号的质量。在曲线图500中示出两个迹线,第一迹线502对应于应用范围402内的光偏置水平的传统系统,而第二迹线504对应于根据本发明的在光发送机122中应用零偏置水平404的系统。根据曲线图500,显然对于低链路损耗值,幅度限制对所传输的信号的影响是一种形式的失真,该失真对本发明的系统比对等同的传统系统导致更低的接收信号质量。然而,在更高的链路损耗值,例如比示例性曲线图500中的20dB大的链路损耗值,包括在与图4B和4C中所示的星座图406、408相对应的点508、510的30dB损耗,实现了更有效地使用可用的光发送机功率的益处。因此,在高水平的链路损耗,使用本发明可实现在功率分配上7dB的改进506。因此,本发明的实施例使得能够实现提供如在自由空间系统中可经历的更大的传输范围和/或更高的可靠性以及对链路损耗的变化的容限的光传输系统。FIG. 5 shows a
如前所述,在本发明的范围内有可能作出许多变形,因此在图6中示出了一种可能的可替代布置600。可替代的传输系统600以与示例性系统100相似的方式采用OFDM技术,然而仅使用奇数编号的OFDM通道来传输信息。在这点上,应注意与此处说明和描述的具体实施例相关地采用的规定将通道号零分配给最低频率的OFDM通道,从而奇数编号的通道包括相对应的相邻的更高频率通道(即通道1),及之后的每个间隔的更高频率通道(即通道3、5、7等)。为了避免与一般情况相关的疑惑,该规定可以如下以更精确的数学术语来表示。N点逆DFT将N个(一般为复数的)频域样本(X(0)...X(N-1))变换成N个(一般为复数的)时域样本(x(0)...x(N-1)),其中,第k个时域样本的值由下式给出:As previously mentioned, many variations are possible within the scope of the invention, and thus one possible
根据该公式,奇数编号的通道是逆DFT的输入X(m),其中m是奇数。According to this formula, the odd-numbered channels are the input X(m) of the inverse DFT, where m is an odd number.
为了简化这里的讨论,在图6中仅示出了两个并行信息输入608,分别对应于奇数编号的通道1和3,然而应理解如之前参照图1所述,在每个数据块中可包括更多的信息位。To simplify the discussion herein, only two
在实施例600中,通过映射单元610根据QAM方案对输入信息位进行编码,以产生相对应的符号值612a、612b。这些符号的复共轭值由共轭器613生成,这确保了IFFT 616被提供适当的输入以生成实数值的时变输出信号。符号值612a、612b及其相对应的复共轭被提供给IFFT 616的奇数编号的输入,而对该例中的偶数编号的输入以及其它未使用的输入提供零值650。输入的符号由组件614以与之前关于图1中的组件114所述相同的方式处理。得到的时变信号被应用于光发送机622,且相对应的不对称限幅的信号通过光通道106被传输到光检测器624。In the
接收信号以与之前参照图1所述相同的方式由频域均衡器组件626处理。与所传输的符号值612a、612b相对应的值634a、634b被输入到均衡器库636,以产生均衡的符号值638a、638b。然后由解调器库640对均衡的值进行解码,以恢复所传输的信息位642。The received signal is processed by frequency
在实施例600中,丢弃与到IFFT 616的零输入值相对应的FFT 632的输出值,例如652。然而在可替代的实施例中有可能使用与零输入值相对应的所接收的符号值,例如作为用于监控和/或优化失真水平或改进接收信号的解调和解码的可靠性的反馈机制。In
可替换实施例600与实施例100相比具有减小的带宽效率,因为仅利用多载波OFDM信号的每隔一个的副载波。然而可看到作为所传输的光信号的幅度限制的结果而生成的失真产物仅在与偶数编号的副载波相对应的频率处产生干扰分量。因此,可替换实施例600能够提供比实施例100产生的信号具有更高质量的接收信号。
还可以看到可通过利用不使用可用的带宽的低频部分的OFDM传输来实现由于不对称幅度限制所导致的失真产物的影响的相似减小。例如,通过将电学OFDM信号调制到具有适当选择的频率的RF载波上,可提供与OFDM信号的RF带宽相对应的低频“防护频带”,使得例如一阶差频失真产物(first-order difference-frequency distortion product)基本上在接收信号带宽外生成。基于失真的容许水平,可接受更窄的防护频带,例如具有OFDM信号的RF带宽的50%,同时仍较大地减少了失真产物对所传输的符号的影响。该布置又提供了可替代的实施例,其中接收信号的质量可以以减小的带宽效率为代价而得到改善。使用低频防护频带的另一个优势-尤其是在采用自由空间光传输时-是来自如包括白炽灯和荧光灯的周围照明的低频光源的干扰在接收机中可基本上被抑制。It can also be seen that a similar reduction in the impact of distortion products due to asymmetric amplitude limitation can be achieved by utilizing OFDM transmission that does not use the low frequency part of the available bandwidth. For example, by modulating an electrical OFDM signal onto an RF carrier with an appropriately chosen frequency, a low frequency "guard band" corresponding to the RF bandwidth of the OFDM signal can be provided such that, for example, first-order difference-frequency distortion products (first-order difference- frequency distortion product) is basically generated outside the received signal bandwidth. Based on the tolerable level of distortion, a narrower guardband may be acceptable, for example having 50% of the RF bandwidth of the OFDM signal, while still greatly reducing the impact of distortion products on the transmitted symbols. This arrangement again provides an alternative embodiment in which the quality of the received signal can be improved at the expense of reduced bandwidth efficiency. Another advantage of using a low frequency guardband - especially when free space optical transmission is employed - is that interference from low frequency light sources such as ambient lighting including incandescent and fluorescent lamps can be substantially suppressed in the receiver.
图7示出了两个波形702和704,这两个波形702和704示出了用于生成用在本发明实施例中的不对称时变电信号的又一可能的技术。波形702、704通过组合频率副载波对而生成,以产生可以以合适的偏置水平应用于光发送机的相对应的不对称波形。通过构造如图7所示的不对称波形,有可能提供本发明的如下实施例:在光发送机中可应用低偏置水平,以生成包括不对称限幅的传输信号的光信号,而不产生信号的相对应的削波和相关联的失真。同样由于需要使用多个副载波以生成波形,例如702、704,所以以带宽效率为代价实现了接收信号质量的改进。Figure 7 shows two
在本发明的一些实施例中,代替OFDM,还有可能利用如脉冲位置调制(pulse-position modulation,PPM)的可替换的调制技术。In some embodiments of the invention, instead of OFDM, it is also possible to utilize alternative modulation techniques like pulse-position modulation (PPM).
本领域的技术人员还应理解,实施例100和600示例了本领域中公知的用于将复数值的符号映射到由强度调制的光源提供的连续的实数值的通道上的两个一般的方法。一个该方法包括在电学域将复数值的实部和虚部正交调制到RF副载波上,然后以得到的实数值信号调制光源。第二个方法包括对发送机的IDFT例如616提供适当的共轭对称输入,或者可替代地实现等同的实数值变换,以生成可用来调制光源的实数值基带信号。应理解这些方法及其等同方法都落在本发明的范围内。Those skilled in the art will also appreciate that
此外,尽管说明了提供单个光发送机和单个光接收机的本发明的实施例,但是应理解本发明适于应用在采用多个发送机和/或接收机的系统中,如MIMO(多输入多输出)光无线系统。实际上,本发明可在该系统中提供特别的优势,因为传统的强度调制的光学系统不能从已知在RF无线系统中可实现的多个源和/或接收机的使用获得益处。Furthermore, although embodiments of the present invention have been described that provide a single optical transmitter and a single optical receiver, it should be understood that the present invention is suitable for use in systems employing multiple transmitters and/or receivers, such as MIMO (Multiple Input Multiple output) optical wireless system. Indeed, the present invention may provide particular advantages in this system, since conventional intensity modulated optical systems do not benefit from the use of multiple sources and/or receivers known to be achievable in RF wireless systems.
例如,在一个可能的布置中多个源(例如多个激光器或LED)可用来发送相同的信号。得到的多个光信号然后可在一个或多个光检测器处被接收、均衡和重组合以在从任何单个源接收的信号的强度是可变的或由于遮蔽(shadowing)而受损的环境中提供改进的可靠性。类似地,当在任何一个检测器处接收的信号的强度是可变的或者受损时,使用多个检测器以及一个或多个源使得能够提高可靠性。For example, in one possible arrangement multiple sources (eg multiple lasers or LEDs) could be used to send the same signal. The resulting multiple optical signals can then be received, equalized, and recombined at one or more photodetectors for use in environments where the strength of the signal received from any single source is variable or compromised due to shadowing. Provides improved reliability in . Similarly, the use of multiple detectors and one or more sources enables increased reliability when the strength of the signal received at any one detector is variable or impaired.
在又一MIMO布置中,多个光源可用来将不同的信息流发送到包括多个光检测器的接收机。已经研发了多种方法来在这种系统中进行空间复用,包括BLAST编码、Alamouti编码等。该方案一般在富散射(richscattering)环境中最有效,且无线光系统在这点上特别有优势,因为不像典型的无线电系统,自由空间光路可包括典型的建筑内应用中的大量反射。In yet another MIMO arrangement, multiple light sources may be used to send different information streams to a receiver comprising multiple light detectors. Various methods have been developed to perform spatial multiplexing in such systems, including BLAST encoding, Alamouti encoding, and others. This approach is generally most effective in rich scattering environments, and wireless optical systems are particularly advantageous in this regard because, unlike typical radio systems, free-space optical paths can include a large number of reflections typical of in-building applications.
包括多个发送机和接收机的进一步的布置包括如下布置:其中多个器件通过共享光介质,如自由空间环境,使用本领域已知的许多适当的多址方法中的任何一个来进行通信。例如,不同的通信器件对可通过共同的光介质,使用根据本发明的方法和设备,结合如频分多址(FDMA)、时分多址(TDMA)、正交频分多址(OFDMA)、码分多址(CDMA)、多载波CDMA(MC-CDMA)等的多址方法来相互通信。Further arrangements comprising multiple transmitters and receivers include arrangements in which multiple devices communicate over a shared optical medium, such as a free space environment, using any of a number of suitable multiple access methods known in the art. For example, different communication device pairs can pass through a common optical medium, using the method and device according to the present invention, combining such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Multiple access methods such as Code Division Multiple Access (CDMA) and Multi-Carrier CDMA (MC-CDMA) are used to communicate with each other.
本发明范围内的其它有利的变形包括使得能够实现在传输距离、可靠性、效率和/或容量上的进一步改进的变形。例如,本发明的一些实施例可被配置成避免在经受或导致增加的干扰、噪声或其它形式的劣化的频率的传输。在本领域已知用于实现如下通道的改进的利用的技术,所述通道呈现频率相关的干扰、噪声、衰减或衰退,特别是与OFDM一起使用。例如,可完全避免经受高水平干扰或减小的信噪比的副载波,或者使用该副载波来传输具有减小的信息内容(即更少的信息位数量)的符号。Other advantageous variants within the scope of the invention include variants enabling further improvements in transmission distance, reliability, efficiency and/or capacity. For example, some embodiments of the invention may be configured to avoid transmission at frequencies that experience or result in increased interference, noise, or other forms of degradation. Techniques are known in the art for achieving improved utilization of channels exhibiting frequency-dependent interference, noise, attenuation or fading, especially for use with OFDM. For example, subcarriers that experience high levels of interference or reduced signal-to-noise ratios may be avoided entirely, or used to transmit symbols with reduced information content (ie, a smaller number of information bits).
在本发明范围内使得能够实现传输性能的改进的又一种技术是以高功率、高数据速率脉冲传输数据块,而不是在较低的功率和数据速率的连续传输。该技术可导致在光检测器处信噪比的整体改善,这可用来实现容量和/或传输距离的整体提高。Yet another technique within the scope of the present invention to enable improvements in transmission performance is to transmit blocks of data in bursts at high power, high data rates, rather than continuous transmission at lower power and data rates. This technique can lead to an overall improvement in the signal-to-noise ratio at the photodetector, which can be used to achieve an overall increase in capacity and/or transmission distance.
因此,对于本领域的技术人员将很明显有可能作出本发明的许多变形,且本发明不限于在此所述的特定实施例。本发明的范围由所附权利要求限定。Thus, it will be apparent to those skilled in the art that many modifications of the invention are possible and that the invention is not limited to the specific embodiments described herein. The scope of the invention is defined by the appended claims.
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- 2006-09-01 CA CA002621251A patent/CA2621251A1/en not_active Abandoned
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Also Published As
Publication number | Publication date |
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EP1920544A1 (en) | 2008-05-14 |
JP2009507405A (en) | 2009-02-19 |
US20090220239A1 (en) | 2009-09-03 |
NZ566407A (en) | 2011-04-29 |
US8107826B2 (en) | 2012-01-31 |
WO2007025346A1 (en) | 2007-03-08 |
KR20080064804A (en) | 2008-07-09 |
EP1920544B1 (en) | 2016-08-24 |
CN101300758B (en) | 2013-04-24 |
EP1920544A4 (en) | 2011-06-15 |
CA2621251A1 (en) | 2007-03-08 |
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